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Full text of "Text Book Of Mechanical Engineering"


'!                       in8                      Appendix VL

Boosters are motor-dynamos applied to automatically in-
crease or ' boost' the voltage of a line already supplied with
current. There are two principal uses for boosters: (i) when a
storage battery requires high voltage for charging during light
load at a station,, and a booster is put in series between the main
generator and the cells to raise the voltage to the required
amount, as in the figure; (2) when the inevitable voltage drop
in long lines is made up in a similar manner.

Motor Starters.—On starting a motor the current would
reach an excessive value if it were turned on to the motor
instantly. To prevent this a multiple-step rheostat is used to
keep the current to a reasonable value, the surplus being absorbed
by resistances that are cut out one by one till full speed be
attained. Referring to Fig. 986, there are three terminals A, B,
and c on the switch board, connected respectively to the main
supply s, the field winding, and the armature. Also B is coupled
to the field switch, and c to the armature switch, each of which
is supplied with a number of resistances. In the figure the
switch arm is shewn on the vulcanite insulators vv, and current
is cut off. Moving the, arm downward, the field winding is
gradually supplied as RJ and R2 are cut out, after which the
armature resistances R8 to R7 are similarly eliminated, the motor
speed being finally regulated on the armature circuit.

J?p, 5$$ and 961. Electric Aceumtilators. —Secondary
cells have improved considerably; Their efficiency is usually
reckoned on< the combined charge and discharge. Stated on
what happens to the cell itself, without regard to the engine and

dynamo :—

f^>               rr- •               ampere-hours discharge        ,         0  0/

Current efficiency = ~~^^^^^^ = about 89%

T-,              n- *                 Watt-hours discharge          ,  -   ,  y »f

Energy efficiency -     Watt4^ dEgT  - about 60%

The reason for the lower result in.the second case is the decrease
of voltage per cell from 2*14 to 1*9 volts between charge and
discharge. Naturally, however, the second statement is the one
having most value for the engineer.